ORA:03113 end-of-file on communication channel while running a query

Hi ,

I am running this query but but I am getting ORA:03113  end-of-file on communication channel 

  1   select cps73_outlet_code, cps73_order_number
  2   from tps73_logical_orders a
  3   where a.cps73_status = 0
  4   and a.cps73_order_number = nvl(null, a.cps73_order_number)
  5   and a.cps73_steps = 1
  6   and rownum < 1000 + 1
  7   and exists ( select 1
  8   from tps701_logical_details t701
  9   where t701.cps701_order_number = 'ABC1'
 10   and t701.cps701_processed = 'N'
 11   and t701.cps701_pre_ind = 'Y')
 12*  order by a.cps73_date_ordered
SQL> /
 select cps73_outlet_code, cps73_order_number
*
ERROR at line 1:
ORA-03113: end-of-file on communication channel 

what are the possible reasons .